The present invention relates to a system for wireless data transmitters and receivers of the type disclosed below, and to a method for downconversion using the modulated signal on the transmitter side.
More specifically, the present invention relates to wireless data transmitters and receivers, where transmission and reception occur at the same time, but at different frequencies, i.e., FDM (frequency division multiplex) to achieve full duplex data communication.
In systems for wireless (radio) data communication there are mechanisms for allocation of frequency slots for the transmission and receiver channels. This access mechanism is denoted FDMA (frequency division multiple access). In some systems there is a constant difference in the carrier frequency between a transmitted and received signal. One example of such a system is Inmarsat Standard M. In this case, it is sufficient to have one frequency synthesizer which is capable of serving both the transmitter and the receiver. By using this system cost savings may be achieved.
According to the present invention, further savings are possible if the transmitted signal itself is used for downconversion of the received signal.
Furthermore, it is an objective of the invention to connect it to modulation methods of the constant envelope type, such as CPM (continuous phase modulation), DPM (digital phase modulation) or near constant envelope modulation such as, e.g., the OQPSK type (offset quadrature phase shift keying).
In U.S. Pat. No. 5,444,737 only FM modulation or FSK modulation is described. The present invention is more general and is capable of modelling all types of constant envelope modulation methods which may be described as either digital phase modulation (DPM) or digital frequency modulation (CPM). The present invention may be used for the OQPSK modulation method (offset quadrature amplitude modulation): filtered OQPSK is, at the outset, not constant envelope modulation, but by elimination of the amplitude portion (hardlimiting) in the transmitter, the transmitted signal will become a constant envelope signal. This means that the amplitude variation is eliminated. OQPSK has the characteristic that the losses associated with such hardlimiting are negligible. With the present invention there are described all modulation methods which either per se possess constant envelope or where constant envelope is provided by hardlimiting in the transmitter either by digital signal processing where the amplitude portion is suppressed, or by a hardlimiting output amplifier (HPA).
In the aforementioned US patent an analog data signal is used which is taken from the transmitter section and which is subtracted from the received signal after a frequency discriminator and an appropriate analog delay.
With the present invention it is not necessary to use a frequency discriminator, but the phase portion (the amplitude is neglected) of the transmitted signal is derived directly from the transmitter signal, then sign inverting is carried out, and from this is formed by exponentiation a complex equivalent baseband signal. By multiplying the received complex equivalent baseband signal in the receiver by the complex equivalent baseband signal for the aforementioned derivative of the transmitted signal, there will be compensation for the unwanted phase perturbation which occurs as a result of downconversion with the modulated transmitted signal used as a local oscillator derived from the transmission signal.
In the aforementioned US patent there is used an analog embodiment which can prove to be rather impractical. The present invention prescribes a fully digital embodiment by use of, e.g., a DSP (digital signal processor).
In the US patent a fixed delay is utilized in an analog execution with RC elements to compensate for the delay in the TX/LO loop. This is not very practical, and with the present invention this is carried out digitally with the aid of a digital Lagrange interpolating filter. Using an LMS algorithm (LMS=least mean square) the delay is either calculated during the production phase, thereafter being set as a fixed value in the radio transceiver, or the delay may be calculated continuously during operation of the radio transceiver with the same as algorithm (adaptive algorithm). In the first case, the LMS algorithm is a part of the production set-up, and in the latter case this algorithm is a part of the radio equipment. This method provides for increased flexibility and a considerably more accurate compensation of the modulation of the signal derived from the transmitter in the event of component variations, temperature drift and aging which may occur in the outer signal path, in contrast to what is previously known.
The aforementioned effects are brought about with the aid of a system of the type introduced above, the characteristic features of which are set forth below, and a method of the type introduced above, the characteristic features of which are set forth below.